HONG KONG — All but one of Hong Kong’s more than 18,000 taxis now burn liquefied petroleum gas (LPG), but one smog-causing pollutant soars if those vehicles’ catalytic converters are not regularly replaced.

Municipal governments all over the world, particularly in developing countries with rapidly growing fleets of cars and choking air pollution, have been rushing over the last few years to force taxis and buses to switch to burning LPG and compressed natural gas (CNG), frequently offering subsidies for them to do so.

But one of the early leaders of the trend, Hong Kong, has said that the city’s shift over the last decade to almost complete dependence on LPG for light commercial vehicles had produced unintended consequences.

Concentrations of nitrogen dioxide, one of the most important contributors to smog, surged by a fifth in Hong Kong’s air from 2008 to 2012, and a team of local and international scientists have traced the cause to LPG-fuelled vehicles, Hong Kong environmental regulators said at a recent news conference.

The problem lies in the catalytic converters on the taxis and minibuses, said Ms Christine Loh, the Under Secretary for the Environment. Unless replaced every 18 months for cars and light buses that are driven nearly around the clock, the catalytic converters become fouled, and the vehicles begin emitting extremely high levels of pollution.

“The LPG vehicles, which are supposed to be cleaner, are spewing out very high levels of nitrogen,” she said. Although natural-gas-powered vehicles have not been deployed on a large scale in Hong Kong, they would pose the same problems, she added.

As a result, the Hong Kong government in the coming months will pay for the free replacement of catalytic converters on the city’s entire privately owned fleet of roughly 18,000 taxis and several thousand minibuses, said Ms Loh. Mr Pang Sik-wing, the city’s Principal Environmental Protection Officer for Air Sciences, said the replacement effort would cost about HK$10,000 (S$1,603) per vehicle.

After the first free replacement, taxi and minibus owners will be responsible for replacing catalytic converters every year and a half at their own expense.

Hong Kong will deploy five mobile sensor systems next year to measure the pollution from passing vehicles and automatically send notices to the registered owners of any vehicle exceeding emission standards, requiring them to take their vehicles for repairs or risk losing their vehicle licences.

“We will strictly enforce the emission standard,” Mr Pang said at a news conference on Friday with Ms Loh.